XML-Lua highlight

This replaces the xml highlighter with one that supports inner lua strings. This is for editing OpenITG/NotITG/Stepmania .xml files that have embedded lua.

If you use vim-plug, you can install by adding this to your init.vim:

Plug 'XeroOl/xml-lua.vim'

Or similar for other plugin managers. Otherwise, just add the syntax/xml.vim file into your vim's syntax folder.